1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter preferably for use in a communication apparatus.
2. Description of the Related Art
In general, surface acoustic wave (SAW) filters are used for communication apparatuses such as cellular telephones. As known in the art, various frequency bands are used depending upon the type of cellular telephone, and various frequency adjusting methods are used to accommodate SAW filters to corresponding frequency bands.
For example, one known method of adjusting frequency bandwidths is to uniformly apply an insulating film to an entire surface of a piezoelectric substrate of a SAW filter that contains interdigital electrodes, and to etch the insulating film so as to provide a thickness according to a desired frequency. A process of manufacturing a SAW filter according to such a method is described below.
FIGS. 1A to 1D are views illustrating a series of procedures to manufacture a known SAW filter.
As shown in FIG. 1A, first, a piezoelectric substrate 11 made of LiTaO3 or LiNbO3 is prepared. After the substrate 11 is polished, a resist pattern 40 formed of a photoresist is applied on a surface of the substrate 11 using a well-known technique. Then, a metal film 41 made of Al or Au having a high conductivity is deposited thereon by a technique such as vapor deposition to provide a predetermined thickness.
As shown in FIG. 1B, the resultant product is immersed into a solvent, and is then irradiated with ultrasonic waves to simultaneously melt, or simultaneously peel off, and therefore remove the resist pattern 40 and the metal film 41 that overlies the resist pattern 40.
As a result, as shown in FIG. 1C, an electrode 42 having a desired pattern is formed by the remaining metal film. The electrode 42 corresponds to interdigital electrodes and reflectors.
Then, as shown in FIG. 1D, on the entire substrate 11 including the electrode 42, an insulating film 43 made of a material such as SiO2 having a different etching rate from the electrode 42 is deposited and adhered by sputtering or chemical vapor deposition (CVD) so as to provide a predetermined thickness.
A wafer prober is used to apply an electrical signal to the electrode 42 to measure a frequency, and to determine a difference between the measured frequency and the target. In order to make the difference zero, the entire surface of the insulating film 43 is etched by a method such as dry etching while monitoring the etching time to adjust frequencies.
A case where the foregoing typical frequency adjusting method is applied to a so-called ladder type SAW filter is described with reference to FIGS. 2 and 3. The ladder type SAW filter includes at least one series arm resonator constituting a series arm, and at least one parallel arm resonator constituting a parallel arm.
In FIG. 2, an exemplary frequency characteristic of the ladder type SAW filter is indicated by (a), and an exemplary impedance characteristic of the ladder type SAW filter is indicated by (b).
As indicated by (a) and (b) in FIG. 2, if the resonant frequency frs of the series arm resonator and the anti-resonant frequency fap of the parallel arm resonator are set to match each other, a band-pass filter having a central frequency f0 is obtained. The bandwidth thereof depends upon the difference between the resonant frequency frp of the parallel arm resonator and the anti-resonant frequency fas of the series arm resonator.
FIG. 3 is a graph showing the shift amount of the resonant frequency and the anti-resonant frequency of a one-terminal-pair surface acoustic wave (SAW) device before and after an SiO2 film is deposited. This graph shows the mean data of samples where the thickness of an Al electrode is 7.2% of the average wavelength of the resonant frequency and the anti-resonant frequency, and where an insulating film made of SiO2 is deposited so as to provide a film thickness of 1.7% of the average wavelength of the resonant frequency and the anti-resonant frequency.
If the insulating film is deposited over the series arm resonator and the parallel arm resonator so as to provide a uniform thickness, the resonant frequency and the anti-resonant frequency of each of the resonators are shifted down, as shown in FIG. 3. As is known, in this case, the anti-resonant frequency is shifted down more than the resonant frequency.
Normally, the wavelength of series arm resonators is set lower than that of parallel arm resonators, and the lower the wavelength, the greater the shift amount of the frequencies. If an insulating film is deposited with a uniform thickness, the frequencies of series arm resonators are thus shifted down compared to the frequencies of parallel arm resonators.
Accordingly, when an insulating film is deposited, the bandwidth of a SAW filter is narrowed because it depends upon the difference between the resonant frequency frp of a parallel arm resonator and the anti-resonant frequency fas of a series arm resonator, and the desired characteristic may not be obtained.
In order to avoid such a problem, in the related art, an insulating film is deposited two times, i.e., a first time for a series arm resonator and a second time for a parallel arm resonator. The insulating film is deposited with different thicknesses so that the shift amount of the frequencies may be substantially the same, before being etched. Therefore, the desired frequency adjustment is achieved.
However, ladder type SAW filters experience problems if the above-described frequency adjustment is performed.
The above-described approach requires two separate depositions of an insulating film. This increases the number of procedures and the time required, resulting in an increased production cost.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter which only requires one deposition of an insulating film in order to achieve the desired frequency adjustment without degrading the frequency bandwidth if it is of the ladder type.
According to one preferred embodiment of the present invention, a surface acoustic wave filter includes a piezoelectric substrate, and a plurality of one-terminal-pair surface acoustic resonators disposed on the piezoelectric substrate. Each of the one-terminal-pair surface acoustic resonators includes interdigital electrodes disposed on the piezoelectric substrate, and an insulating film deposited on and adhered to the interdigital electrodes. At least one of the plurality of one-terminal-pair surface acoustic resonators is a series arm resonator, and at least one of the remaining one-terminal-pair surface acoustic resonators is a parallel arm resonator. The series arm resonator and the parallel arm resonator are coupled in a ladder arrangement.
The electrode duty of the parallel arm resonator is greater than the electrode duty of the series arm resonator when the electrode duty of a one-terminal pair surface acoustic resonator is defined by the following equation (1):
electrode duty=2xc3x97W/xcexxe2x80x83xe2x80x83(1) 
where xcex denotes the wavelength of the one-terminal-pair surface acoustic wave resonator, and W denotes the line width of an interdigital electrode.
This prevents degradation of the frequency bandwidth when an insulating film is deposited. Thus, an insulating film having a greater thickness can be deposited, and it functions as a protection film, thereby increasing reliability.
The thickness of the insulating film may be adjusted so as to provide a predetermined frequency characteristic.
Preferably, the electrode duty of at least one series arm resonator is about 0.5 or less. As a result, more efficient frequency adjustment can be achieved. Furthermore, the shift amount of the frequency bandwidth for frequency adjustment is less, thereby achieving the frequency adjustment while satisfying the desired filter characteristic.
The surface acoustic wave filter has preferably attenuation standards in the vicinity of a high frequency region and a low frequency region of the pass band.
The surface acoustic wave filter according to a preferred embodiment of the present invention may be suitably implemented as a GSM-1900 or DCS filter for use in a communication apparatus in which a narrow or wide bandwidth may cause defects.
In another preferred embodiment of the present invention, a communication apparatus includes the surface acoustic wave filter in accordance with the preferred embodiment of the present invention described above.
Therefore, the communication apparatus incorporating such a surface acoustic wave filter is inexpensive and has the desired frequency characteristic.
Other features, elements, steps, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments with reference to the attached drawings.